I don’t think erectile dysfunction post op is less likely in men that had good pre-op function although obviously a man who already had ED is not going to see a dramatic improvement after RP. It all comes down to how much of the nerve bundles can be saved, how battered / burnt / bruised they are during the op and a bit of luck. Some men have total nerve sparing but the temporary damage basically leads the nerve bundles to pack up and retire permanently. Other men have significant nerve removal or major incidental damage and yet a small amount of nerve is able to regenerate. In more cases, the man regains physical ability to get an erection (sooner or later) but the emotional impact of diagnosis, cancer treatment, worries about the future, fear of failure and questioning what it all means for their sense of maleness cause ED.
In our house, we are in the third camp.